Study On Preparation And Performances Of Pem Sulfonated Poly(phenylene Oxide) For High Temperature Pemfc

Proton exchange membrane fuel cells (PEMFC) are attracting people's attention as clean power sources for vehicular transportation and their potential applications in electrochemical devices. PEMFC has been considered as a promising energy source because of their high efficiency at low temperatures, easily constructed and environment friendly. As one of key components for the PEMFC system, the study of PEM is very important in the development of PEMFC. The commercially available Nafion membranes produced by DuPont have been extensively employed in PEMFC. However, there are still many shortcomings will bind their wider application in PEMFC, including limited operation temperatures, high methanol permeability and high cost. However, a number of advantages were reported for PEMFC to be operated at high temperature (120-200°C), such as improved CO tolerance of the platinum electrode, simplified water and heat managements, and increased the energy efficiency. Thus the preparation of new PEMs which providing a good performance at high temperature and low relative humidity is still one of the critical challenges in the area of fuel cell technologies.Due to its good thermal stabilities and high performance, aromatic polymers have been widely employed as PEMFC materials. In this paper, sulfonated poly(phenylene oxide) (SPPO) was prepared through sulfonating poly(phenylene oxide) with chlorosulfonic acid, and its comprehensive properties were investigated. The structure of SPPO was confirmed by FTIR and 1HNMR, and the influences of the factors affecting the sulfonation reaction were studied. In addition, series SPPO membranes with different degree of sulfonation (DS), was also investigated, and the optimum of DS, 35%, was obtained. SPPO-xIm composite membranes with different content of imidazole (Im) from 0.25 to 4 in SPPO (35%DS) polymer matrix were prepared by solution cast film. The relation between doping ratio x (x is the molar ratio of Im to SPPO repeat unit) and membranes properties were studied by means of thermogravimetric-differential thermal analysis (TG-DTA), X-ray diffraction (XRD), dynamic mechanical analysis (DMA), and scanning electronic microscope (SEM); and the investigation of conductivity activation energies of the SPPO-xIm at different relative humidity was also performed. TG-DTA curves reveal that these SPPO-xIm composite materials had a high thermal stability, and all these membranes show a lower swelling degree and moderate uptake. The proton conductivity of SPPO-xIm composite material increased with the temperature, and the maximum proton conductivity of SPPO-xIm composite materials was found to be 6.92×10-3 S/cm at 200°C, 33%RH and x = 2. Under high temperature and low relative humidity condition, the proton conduction was mainly in accordance with"Grotthuss mechanism". Measurement results of DMA showed that the SPPO-2Im composite membranes possess excellent rigidity and tenacity.In this study, the effect of Br?nsted basicity on the proton conductivity of composite PEMs was also discussed. Five acid-base composite materials with different basicity were prepared by doping of SPPO of 35% DS with different Br?nsted bases, such as imidazole (Im), pyrazole (Py), benzimidazole (BnIm), 1-methylimidazole (1-MeIm) and 4-methylimidazole (4-MeIm) respectively. The resulting proton conductivities of the SPPO-heterocyclic composite membranes showed differences at approximately one order of magnitude at a high temperature and 33%RH condition, which reveals that the proton conductivities were related with not only dissociation constant (pKa), but also the molecular structure of basic heterocyclic. Based on SPPO-2Im, SPPO/2Im/nano-23wt%ZrP (zirconium phosphate) organic-inorganic composite membranes were prepared by method of ion impregnation-precipitation. TG-DTA analysis showed that SPPO/2Im/nano-23wt%ZrP has a high thermal stability. SEM pictures showed that the ZrP particles uniformly dispersed in the SPPO matrix and the size of ZrP particles reached nanometer level. XRD spectra showed thatα-ZrP was generated. Water uptake and swelling degree of SPPO/2Im/nano-23wt%ZrP was higher than that of SPPO-2Im membrane. This SPPO/2Im/nano-23wt%ZrP membrane provided proton conductivity as high as 10-2 S/cm at 120-200°C and 33%RH conduction. The optimized preparation ways were obtained: the reaction temperature was 80°C, the concentration ratio of C Zr4+/C H3PO4was 1, and the reaction time with zirconyl chloride and phosphoric acid was 6h and 12h respectively.The fixation of imidazole in composite membrane was also attempted. Imidazole ring were grafted on alkoxysilane with a nucleophilic reaction to synthesis 2-((3-triethoxysilylpropyl) thio)-imidazole, which was mixed with SPPO to form hybrid composite membranes with fixed imidazole rings by cross linkage. The hybrid composite membranes exhibited a low proton conductivity of approximately 10-6 S/cm. Although it needs further improvement, by comparing with the situation that imidazole dopant gets lost from membrane, it provide new idea of immobilization of imidazole.The above primary exploration indicate that SPPO-2Im,SPPO/Im/ZrP composite membranes can be applied on PEMFC at 120-200°C.